Method of making aluminum foil for fins

ABSTRACT

A method is described for making an aluminum alloy foil suitable for application to fins used in heat exchangers. The method comprises providing an aluminum alloy composition containing about 0.27% to about 0.55% by weight of iron, about 0.06% to about 0.55% by weight of silicon and optionally up to about 0.20% by weight of copper; continuously casting a coiled strip from the molten aluminum alloy; cold rolling the continuously cast coil to a final gauge of about 0.076 mm to about 0.152 mm and partially annealing the aluminum alloy sheet at a temperature below about 260° C., with a maximum overheat of about 10° C. to anneal the aluminum alloy foil substantially without any recrystallization.

TECHNICAL FIELD

[0001] The present invention describes a method of fabricating analuminum foil suitable for application in fins used in heat exchangers,particularly for condenser and evaporator coils.

BACKGROUND ART

[0002] Aluminum foils are popularly used in heat exchangers becausealuminum has very high thermal conductivity. These fins are typicallyfitted over copper tubes and mechanically assembled. As the size of theair conditioner units increases, the fins become longer, and it isimportant that they have sufficient strength so that they can be liftedwithout bending. Low strength can also result in handling damage whenthe coils are bent to form a unit. One way to improve the rigidity ofthe coil is to increase the gauge of the aluminum foil. Since thisalternative is costly, and adds weight, air conditioner manufacturersprefer to use stronger foil.

[0003] The most popular alloy used in this application is the alloy AA1100. It has the composition shown in Table I below: TABLE I Elements WtSilicon + Iron: <0.95 Copper: 0.05-0.20 Aluminum: >99.00 Other elements:<0.05

[0004] When fully annealed, this alloy has very low strength. Forexample, typical yield strength could be between 20.7-41.4 MPa (3-6ksi), and ultimate tensile strength (UTS) could be between 96.5-110.3MPa (14-16 ksi). This alloy is highly formable, with elongationgenerally exceeding 24% and Olsen values above 0.25 in. (6 mm). If theformability is inadequate, the collars formed in this sheet throughwhich the copper tubes are passed can crack in the reflare or in thebody of the collar itself. These cracks are undesirable because thecopper tubes, after passing through the fins, are expanded to form agood joint between the collar and the tube. If the collar is cracked,heat transfer between the fin and the tube deteriorates. “0” temper, AA1100 sheet forms excellent collars and is popularly used in thisapplication. A problem arises when higher strength is desired inapplications such as long fins.

[0005] Typically, AA 1100 alloy formed by direct casting or DC method,hot rolled and then cold rolled to the final gauge of 0.1-0.13 mm(0.004-0.005 in), can be partially annealed. The partial anneal stepinvolves heating the cold rolled sheet at temperatures between 240-270°C. During this time, the strength of the cold rolled sheet decreases andits formability increases. The cold rolling destroys the aluminumstructure completely. When it is heated, the first step involvesrecovery and the second step involves recrystallization. In a typicalanneal, the step of recovery involves a gradual reduction in strengthwhile recrystallization involves precipitous decline in strength. Thetypical desired mechanical properties of a partially annealed sheet areshown in Table II below: TABLE II Yield strength (MPa) 96.5-110.3Elongation (%) 20-24  UTS (MPa) 110.3-124.1 

[0006] The partially annealed material has a structure that is fullyrecovered and has started forming some initial grains (incipientrecrystallization). These grains are small, typically less than 25micron in diameter. This material performs extremely well in finapplication with collar cracks generally below 5%.

[0007] DC casting method, however, is expensive. In recent years, therehas been a trend to go to continuous casting, using belt casters, rollcasters, or other similar equipment. Continuous casters produce an“as-cast” strip that is less than 30 mm in thickness (more generallyless than 25 mm in thickness). Roll casters generally produce a strip of6 mm or less that can be directly cold rolled. Belt casters producestrip that can be either directly cold rolled or may be used inconjunction with an in-line rolling mill that reduces the thickness ofthe as cast slab, after it is solidified but before it cools, to athickness suitable for cold rolling. The hot rolling step in DC castmaterial is preceded by a preheat (homogenization) at around 500° C.This homogenization step is not present in continuous casting method,and thus the thermal history of the two materials is significantlydifferent. As a result, DC cast AA 1100 material produces excellentpartially annealed sheet, whereas the corresponding continuous caster(CC) cast sheet has so far failed to give the desired performance. CCcast material is less formable than DC cast material at equivalentstrength. Attempts to improve the formability (as characterized byelongation and Olsen values) by increasing the anneal temperatureresults in reduction of yield strength significantly below the lowerlimit of 89.6-96.5 MPa.

[0008] Various studies and previous attempts have been made to developimproved methods of making aluminum foils utilizing a single rollcontinuous casting method and an aluminum based alloy composition whichcan be single roll cast, homogenized, cold rolled and annealed toproduce an aluminum foil product. For example, U.S. Pat. No. 5,466,312(Ward, Jr.) discusses a method of making an aluminum foil whichcomprises providing a molten aluminum-based alloy consisting essentiallyof about 0.08 to 0.20 weight percent silicon, about 0.24 to 0.50 weightpercent iron, and about 0.21 to 0.30 weight percent copper, with thebalance being aluminum and inevitable impurities. The aluminum alloycomposition is continuously cast to form a coiled cast strip. The coiledcast strip is homogenized, cold rolled, and followed by a finalrecrystallizing annealing step of 450-650° F. This temperature rangecreates recrystallization in the foil.

[0009] U.S. Pat. No. 5,554,234 (Takeuchi) proposes high strengthaluminum alloy suitable for use in the manufacture of a fin. Accordingto the patent, the aluminum alloy contains at most 0.1% by weight ofsilicon, 0.10 to 1.0% by weight of iron, 0.1 to 0.50% by weight ofmanganese, 0.01 to 0.15% by weight of titanium, with the balance beingaluminum and unavoidable impurities. The patent also discusses a methodof manufacturing a high strength aluminum alloy suitable for use in themanufacture of a fin, which comprises the step of heating an aluminumalloy ingot to 430-580° C., hot rolling the ingot to obtain a platematerial, and applying a homogenizing annealing treatment at 250-350° C.for the stated purpose of causing intermetallic compounds to bedistributed within the metal texture of the alloy.

[0010] U.S. Pat. No. 4,737,198 (Shabel) discloses a method of casting analloy having components in the composition range of about 0.5-1.2% iron,0.7-1.3% manganese, and 0-0.5% silicon by weight, homogenizing the castalloy at temperatures below about 1100° F., preferably below about 1050°F. to control the microstructure, and cold rolling to a final gauge. Thecold rolled alloy is then partially annealed to attain desired levels ofstrength and formability.

[0011] Japanese Patent No. 5-51710 proposes an aluminum foil annealed at150-250° C. in a hot air furnace which carries the foil along on a hotair cushion at a temperature of 350-450° C. Japanese Patent No. 6-93397discusses an aluminum alloy for making a foil and a treatment method toimprove the properties of the foil, including cold rolling, heattreatment up to 400 C., and then process annealing at 250-450 C.,followed by further cold rolling.

[0012] It is an object of the present invention to provide an improvedmethod for producing aluminum alloy foil for heat exchanger fins basedon continuous casting of an AA 1100 aluminum alloy.

DISCLOSURE OF THE INVENTION

[0013] The present invention provides a method for making an aluminumalloy foil for fins used in heat exchangers. The alloy may be an AA 1100type aluminum alloy, such as an aluminum alloy containing about 0.27% toabout 0.55% by weight of iron and about 0.06% to about 0.55% by weightof silicon.

[0014] The alloy also preferably contains about 0.05% to about 0.20% byweight copper. This alloy in molten form is continuously cast into analuminum alloy strip, which continuously cast strip is cold rolled to afinal gauge of about 0.076 mm to about 0.152 mm. The cold rolled stripis subjected to a partial annealing treatment at a temperature belowabout 260° C., with a maximum overheat of about 10° C. In this manner,the annealing of the aluminum alloy foil takes place with substantiallyno recrystallization.

[0015] The invention provides a strong yet formable improved aluminumalloy foil suitable for use in making fins for heat exchangers,including condensers and evaporators used in air conditioning equipment.

BEST MODES FOR CARRYING OUT THE INVENTION

[0016] It has been found that the difference between CC and DC castmaterial cannot be explained in terms of the alloy composition. Forinstance, aluminum alloys of various compositions including high and lowFe (0.27-0.55%), high and low silicon (0.06-0.55%), and changes incopper content (0.00-0.12%) were tried but the result was always thesame. The CC cast material was less formable than the DC cast material.For example, the elongation of DC cast material when the yield strengthis 96.5 MPa is around 22%. The corresponding yield strength atequivalent elongation for the CC cast material was around 48.3-62.1 MPa.

[0017] The difference between CC cast and DC cast material can be tracedto the difference in the microstructure of the two partially annealedmaterials. During initial recrystallization, the DC cast material formssmall grains but the CC cast material forms large grains. This may bedue to the fact that fewer recrystallization sites are available in CCcast material due to the presence of these large grains rather than thebulk formability. This was unexpected, as it was always felt within theindustry that the collar cracks were caused by inadequate elongation orOlsen values. This was only partially true. As long as the partiallyrecrystallized material did not contain more than 5% of recrystallizedgrains, preferably not more than 2% of recrystallized grains, collarcracks did not form even when the elongation was only between 16-18%.Thus, for the CC material to adequately function in the fin-application,it was critical to prevent significant recrystallization of the materialduring the partial anneal.

[0018] Further, the presence of large grains in CC material could notonly be correlated to the anneal temperature but also to the overheatprovided in the furnace. Heat head, or overheat, is the differencebetween the metal and air or gas temperatures in the furnace. The air orgas temperature is measured directly by a thermocouple near the heatsource and in the air flow in furnace and the metal temperature isgenerally measured by a thermocouple embedded within the coil in thefurnace. For preventing recrystallization but allowing recovery to takeplace, the anneal temperature should not exceed 260° C., and preferablyshould be between 245-255° C. The overheat should not exceed 10° C.,preferably should be less than 7° C. Under these circumstances, norecrystallization takes place. The anneal time is provided to finishrecovery of the material. The low overheat imposed in the present methodensures the greatest possible uniformity of temperature during theanneal process and consequently the formation of even small amounts ofrecrystallized grains is prevented whilst operating at the highestpossible temperature for recovery.

[0019] When the anneal practices referred to are followed, a CC castmaterial gives a microstructure that is essentially recovered and hasvery few, if any, recrystallized grains. The typical properties of sucha material are shown in Table III below: TABLE III Yield Strength (MPa)93.1-110.3 Ultimate Tensile Strength (MPa) 110.3-124.1  Elongation %16-19  at 0.10 mm gauge

[0020] Although the elongation of this material is significantly lowerthan the corresponding DC cast material, this material performsextremely well in fin applications.

[0021] During the formation of collars, aluminum is stretched by asignificant extent. This depends upon the design of the collar. However,in a typical application, during the reflaring of the collar, the radialstretch could be as much as 20%. This is the main reason why cracksappear during reflaring. If large, recrystallized grains are presentlocally, then these grains stretch much more, being pliable compared tothe rest of the material. Therefore, cracks appear even though the bulkproperties could be excellent. By preventing recrystallization, andoptimizing the anneal practice to give the maximum possible formability,collar cracks are prevented.

[0022] Currently, only DC cast material performs well in thisapplication. By developing a CC cast alternative, the present inventionprovides a much more economical alternative.

[0023] The present invention includes continuously casting a Cu—Fe—Si—Alalloy and fabricating the alloy to a light gauge sheet or foil, e.g.,sheet having approximately 0.076-0.152 mm thickness, followed bycontrolled partial annealing to achieve combinations of strength andformability not achieved by conventional techniques. The partial annealis preferably carried out a batch anneal with the cold rolled sheet incoil form.

[0024] The preferred composition range for the alloy in accordance withthe present invention is shown in Table IV below: TABLE IV Elements Wt %Copper 0.05% to 0.20% Silicon 0.36% to 0.44% Iron 0.39% to 0.47%

[0025] The silicon range of 0.3-0.5 wt % preferably 0.36-0.44 wt % andiron range of 0.3-0.5 wt % preferably 0.39-0.47% are chosen so thatduring the continuous casting process a single intermetallic species(alpha phase) is formed. Since the material does not undergo anysubsequent homogenization process, this prevents the formation ofsurface rolling defects (“smut”) during the cold rolling process.

[0026] Copper in the range given adds strength to the final productwithout causing excessive work hardening during the foil rolling stage.

[0027] The specified alloy is cast using a belt caster and in-linerolling mill to 1.7 mm gauge. The alloy is then cold rolled to the finalproduct gauge. For fin stock applications, the final product gauge is inthe range of about 0.076-0.152 mm. Partial annealing is then employed tooptimize strength and formability. An example of the combined strengthand formability that can be achieved for an annealing temperature of250° C. is shown in Table V below. TABLE V Yield Strength (MPa) 100.0UTS (MPa) 119.3 Elong 18.5 Olsen 5.7 mm

[0028] Another example of the combined strength and formability that canbe achieved for an annealing temperature of 248° C. is shown in Table VIbelow: TABLE VI Yield Strength (MPa) 111.0 UTS (MPa) 225.5 Elong 17.5Olsen 5.8 mm

[0029] The percentage of reflare cracks in both of the examples abovewere the same as in DC material at 0.5%. Only two rows of fin showeddefects in both DC and CC material. Comparison of DC and CC material inthe same rows of fins indicated that the number of defects wereidentical.

[0030] The process of the present invention has been found to develop afine grained, high strength fin stock alloy with good formability. Thealloy is particularly useful in producing light gauge sheet or foil forfin stock. The process of the present invention does not contain a hotrolling step preceded by a preheat at around 500° C.

[0031] The following example is intended to illustrate the practice ofthe claimed invention and is not to be construed as limiting.

EXAMPLE 1

[0032] An AA 1100 alloy of the following composition was cast using abelt caster and in-line rolling mill to 1.7 mm gauge. The compositionrange for the alloy is shown in Table VII below: TABLE VII Elements Wt %Silicon 0.42% Iron 0.41% Copper 0.06%

[0033] These coils were then cold rolled to 0.10 mm gauge in threepasses. The final coil was annealed with different annealing practiceswith a heat head of 50° C. The annealed coils were tested in fin pressesand reflare cracks were counted and compared with a corresponding DCmaterial (properties, yield strength 100.0 MPa, elongation 22%). Theresults are given in Table VIII below: TABLE VIII Excess Anneal Practicecracks Step 1 Step 2 UTS YS Elong Olsen over Coil Temp ° C. Time Temp °C. Time MPa MPa % mm DC % 1 235 2 258 6 119.8 92.8 18.0 6.0 14 2 235 2262 6 110.3 75.2 22.0 6.1 41.6 3 235 2 262 6.5 106.1 63.4 20.5 6.4 52 4235 2 262 6.5 101.3 52.4 21 7.0 58

[0034] As can be seen from the above data, the reflare cracks generallyincreased with increasing elongation and decreasing yield strength. Whenthese samples were examined optically, the structure revealed presenceof large grains that were partially recrystallized. On the other hand,the DC structure showed only very small grains, if any. The onset oflarge grains was probably caused by the high heat head which wasmaintained in the furnace and which caused a part of the coil to reachtemperatures significantly higher than the target resulting in graingrowth.

[0035] To avoid this and prevent any recrystallization, a new annealingpractice was devised. This involved maintaining a very small heat headin the furnace, not exceeding 10° C. and preferably below 7° C. Theannealing temperature was also brought down to avoid recrystallizationaltogether, as it was felt that this was the main reason for the poorperformance of the CC material. The results are given in Table VIXbelow: TABLE VIX Heat Anneal Practice Head UTS YS Elong Olsen Coil Temp(° C.) Time (hrs) (° C.) MPa MPa % mm 1 250 7 5 119.2 100.0 18.5 5.7 2248 8 5 125.5 111.0 17.5 5.8

[0036] The percentage of reflare cracks were the same in DC material at0.5%. Only two rows of fins showed defects in both DC and CC material.Comparison of DC and CC material in the same two rows of fins indicatedthat the number of defects were identical.

1. A method of mucking an aluminum alloy foil for use in heat exchangerfins which comprises (a) providing a molten aluminum-based alloycontaining 0.27% to 0.55% by weight iron, 0.06% to 0.55% silicon,optionally 0.05% to 0.20% copper and the balance aluminum andunavoidable impurities, (b) continuously casting said molten aluminumalloy into an aluminum alloy strip, and (c) cold rolling thecontinuously cast aluminum alloy strip to a final gauge of about 0.076mm to about 0.152 mm, characterized by partially annealing the aluminumalloy strip at a temperature below about 260° C. with a maximum overheatof about 10° C. to thereby anneal the aluminum alloy foil substantiallywithout recrystallization.
 2. A method according to claim 1,characterized in that the aluminum alloy contains 0.05% to 0.20% byweight copper.
 3. A method according to claim 2, characterized in thatthe aluminum alloy contains 0.36% to 0.44% by weight iron and 0.39% to0.47% by weight silicon.
 4. A method according to claims 1, 2 or 3,characterized in that the foil is partially annealed for a period oftime of less than about 10 hours.
 5. A method according to any one ofclaims 1 to 4, characterized in that the foil is partially annealed at atemperature in the range of about 245° C. to 255° C.
 6. A methodaccording to any one of claims 1 to 5, characterized in that theoverheat during annealing is no more than about 7° C.
 7. An aluminumalloy foil for use in heat exchanger fins produced by a method accordingto any one of claims 1 to 6.